The initial size of the winsxs folder after a fresh installation should be appr. 5GB in Windows 7. It grows when you download programs which bring .dlls (dynamic link libraries which are common subroutines). When you uninstall these programs, the .dlls remain because they can be in use by other programs too. That is something the uninstaller does not know.
And to answer your question - yes a fresh install will reduce the size of the winsxs from your 18GB to appr. 5GB. But if you install many new programs, it will grow again. 18GB is unusually large. My system with many programs installed during the last 4 years has a winsxs of 12GB. I would rather get a larger SSD than worry about the winsxs size.